The invention relates to a process for manufacturing a precursor powder for use in making a varistor and containing dopants which after bending said precursor powder with a basic powder in a ceramic process are distributed in the grains of the basic powder.
It is known from U.S. Pat. No. 4,142,996 to manufacture a precursor powder for use in making a varistor in combining oxy-salts of zinc and dopant additives in water, heating to form a hydrated melt, and heating further to form a dehydrated melt. The melt is spray-dried to form a varistor ceramic powder. A further process for manufacturing a precursor powder for use in making a varistor is described in U.S. Pat. No. 4,540,971, in which zinc oxide is combined with at least a portion of the dopants. In this process, an aqueous solution containing salts of zinc and components is coprecipitated to form a uniform precipitate. The precipitate is then collected by vacuum filtration and is freeze-dried to remove free water and to form the precursor powder.
It is further known to precipitate zinc under hydrothermal conditions. U.S. Pat. No. 1,536,619 describes the precipitation of zinc hydroxide and zinc carbonate from zinc chloride solutions to recover zinc metal values. U.S. Pat. No. 1,863,700 discloses a similar process, although the precipitate is heated in saturated steam at 300.degree. C. to increase the particle size of the precipitate. In U.S. Pat. No. 3,674,476 a hydrothermal treatment step is used to produce a zinc oxide powder for use in a photoconductive layer for electrophotography. In this process, an autoclave treatment of zinc hydroxide or zinc carbonate at a temperature of between 180.degree. and 300.degree. C. is employed to produce a fine zinc oxide powder. A hydrothermal process for producing a doped zinc oxide powder for varistor applications is described by W. J. Dawson in Ceramic Bulletin, Vol. 67, No. 1988, p. 1673-1078. Hereby the goal is to produce a fully precipitated compound containing primarily zinc oxide and varistor dopants including bismuth and manganese oxides. A hydrothermal treatment step was used to accomplish the precipitation.